1. Field of the Invention
The present invention pertains to a method of manufacturing a sheet material and a method of manufacturing a circuit device using the same, and, more particularly to a method of manufacturing a sheet material which enables the formation of plated films of uniform thickness over a conductive foil, and a method of manufacturing a circuit device using the same.
2. Description of the Related Art
A lead material coated with a plated film over a surface of a conductive element exhibits excellent conductivity and mechanical strength of the conductive material. The lead material is a high-performance conductor exhibiting corrosion resistance and good solderability of the plating material. Therefore, this material is very often used in electrical or electronic equipment fields, electric cables fields such as various kinds of terminals, connectors, or leads.
When mounting semiconductor elements on the conductive elements and wire-bonding thereto, if hot dipping or electroplating is formed over the surface of the conductive element, the solderability of the conductive element can be improved. Herein, Au or Ag films, etc. are employed as the plated film.
A semiconductor device using an Ag plating to a bonding pad and a die pad will now be described with reference to FIG. 15.
This semiconductor device comprises a lead 3 having an Ag plating 8 at the tip, an island lead 4 having the Ag plating 8 at a portion of an island 2, a semiconductor element 1 mounted on an island 2 via Ag—Si eutectic crystal, metallic wires 5 for electrically connecting an electrode of the semiconductor element 1 and a lead 3, and an insulating resin 6 sealing the entire semiconductor device. By mounting the semiconductor element 1 on the island 2 via the Ag—Si eutectic crystal, heat radiation of the semiconductor element 1 may be improved.
Next, one of the steps of manufacturing the semiconductor device which is forming the Ag plated film will be described.
Referring to FIG. 16A, the island lead 4 having the island 2 and the lead 3 are supplied as one frame as a lead frame 7. The lead frame 7 is formed of a conductive material such as Cu, wherein a plurality of island leads 4 and leads 3 are provided.
Referring to FIG. 16B, one portion of the lead frame 7 is covered with a clamper 9. Herein, one portion of the lead frame 7 is at least a portion becoming the die pad of the island 4 and a portion becoming the bonding pad of the lead 3.
The clamper 9 will now be described. The clamper 9 has a part covering a front side of the lead frame 7 and a part covering a back side (not illustrated). Further, since it has a hollow structure, the interior of the clamper 9 is a hermetically sealed space. Accordingly, if a portion of the lead frame 7 is inserted as mentioned above, only the portion forming the Ag plating 8 is exposed in this space. Herein, hatched parts indicated with numeral 9 in FIG. 16B show parts where the upper and lower clampers are in mesh and represent the area where the hatched parts are exposed.
An Ag plating liquid is injected into this space and the Ag plating can be formed by the electroplating method.
The above describes the Ag plating method, and there are cases when Au plating is used. When Au is used as a plating material, due to cost considerations, a Ni plated film is first formed, and then the Au plated film is formed. Because the Ni plated film acts as a substrate, it is possible to prevent Cu which is the material of the lead frame 7 from dispersing into the Au plated film, and accordingly to prevent the Au plated film bondability from deteriorating.
Another method of forming a plated film will be further described. By using an electrodepositing resist, a plating resist is selectively formed such that surfaces of leads only of portions to be pads are exposed. In this case, the plated film is formed by immersion plating. By this method, the plated films can be formed only at the parts to be the pads, and the plating resists are removed in a post-step.
After the plated film is formed, and performing the steps of die-bonding, wire-bonding and resin sealing, the semiconductor device is completed as shown in FIG. 15.
However, as shown in FIG. 15, in the lead 3 and the island 2, the plated films are also formed on the back surface and the side surface. Since adhesive force between the plated film and the sealing resin is weak, there is a problem that mechanical strength of the semiconductor device goes down.
Further, when the plated film is formed by immersion plating, there is another problem that the plated film remote from an electrode provided on the lead frame is formed to be thicker than the plated film near to the electrode.
For performing the Au plating, the Ni plated film is formed as a groundwork, on which the Au plated film is formed. Accordingly, there is a further problem that the process becomes complicated.
If using the electrodepositing resist to selectively form the plated film, a problem occurs that positioning precision of the plated film is low.
Still further, in case of using a strong alkaline plating liquid, materials of the electrodepositing resist peel. Accordingly, another problem is that the Ag plating using the strong alkaline plating liquid cannot be performed.